There are fashions in diseases, as in anything else. It’s understandable that a new, infectious and life-threatening malady could preoccupy us, such as cholera in the 19th century or Ebola in recent times.

It is harder to see why a panic erupts around a diagnosis that’s a century old, but a telegenic celebrity death can help. When the singer Karen Carpenter died aged 32 in 1983, her heart gave out because of complications due to anorexia. Her death is widely credited with pushing eating disorders into the public consciousness.

Karen Carpenter was not the first famous young woman to starve to death. Sarah Jacob, “the Welsh Fasting Girl”, was once a national craze across Britain. She died at her parents’ farm in December 1869 in front of a team of nurses who had been sent from London to Carmathenshire to monitor her.

Sarah was believed by her family and her local clergyman to eat nothing at all. Her parents agreed to have her watched to make sure she was not secretly eating, but their faith in her was strong enough that they refused to have her force-fed.

The Welsh Fasting Girl was a national craze in the 19th century.Wellcome Trust

As with other fasting girls, her alleged ability to live without food was taken by her supporters as a sign of special spiritual status, and seen by materialist physicians as evidence of hysteria and deceit.

Did Sarah Jacob, like Karen Carpenter, die of anorexia?

The diagnostic label “anorexia nervosa” was not coined until shortly after Sarah Jacob died, but of course a disease can exist prior to being named. She did not have all the symptoms associated with the modern diagnosis, but most mental disorders vary from patient to patient.

Anorexia is often seen as an expression of will – an assertion of autonomy and control by a young woman who is engaged in a battle with her family and therapists. If that’s the crucial point about anorexia then maybe Sarah Jacob was anorexic. Her fast turned her whole domestic world upside down and she maintained it right to the end.

In her 1988 history of anorexia, Fasting Girls, Joan Jacobs Brumberg, noting the presence of the medical team watching in her room, asserted that Sarah was “killed by experimental design”. But maybe she died of pride.

If the assertion of will, over both one’s own appetite and the authority of others, is the heart of anorexia, then perhaps we can push its history back further. In Holy Anorexia (1985), Rudolph Bell argued that anorexia shaped the lives of many medieval saints and other holy women, who ate next to nothing.

Saint Catherine of Siena fasted for days, far beyond what was expected of even the most pious young women in 14th-century Italy. She did so even when the male priests she was supposed to defer to expressly told her to eat something, on the grounds that her spiritual husband, Jesus himself, outranked them.

For Bell, it is Catherine’s assertion of her will – she sent angry letters to the Pope – that marks her out and puts her in a long line of anorexics extending to the present day.

Brumberg attacks Bell for assuming that female psychology has not changed over the centuries and that the past and present are the same.

But that’s unfair. It is certainly possible to acknowledge that both psychology and culture have changed dramatically over the years while also thinking that two people share enough relevant symptoms and personality features to justify applying the same diagnostic label to them both even if they lived centuries apart.

But obviously not just any remote similarity is enough, so how can we decide?

Archaeologists can find on ancient skeletons the traces of familiar diseases, but there is no physical marker to point to that would decide whether a mental illness was present in the middle ages.

Clearly, young women (and men) have been dramatically restricting their calorie intake for centuries, but not all the symptoms of modern anorexia have always been present, and some saintly behaviours are no longer associated with eating disorders.

Similarly, melancholy has a very long history, and many scholars see modern depression as essentially the same thing.

But modern clinical depression has dropped the distinction between melancholy, which has no obvious cause, and ordinary sadness, which is a reasonable response to the tragedies of life. “Depression” pathologises parts of our mental life that “melancholy” treated as normal – is it the same disease, or not?

Well, if you think mental illness is above all a problem with a neurological system, then there might seem to be an easy answer. The disease label refers to what is going wrong within your brain, and the cultural context just supplies the input and output.

Take an anorexic brain and plug it into 14th-century Italy and you get one set of symptoms. Plug it into modern Western societies and you get another. The different symptoms are reflections of different cultures acting on the brain.

Joel and Ian Gold, in Suspicious Minds, have discussed the emergence of what they consider to be a new form of psychopathology – the “Truman Show delusion” – in which, like the hero of the movie of that name, subjects imagine themselves as the star of a reality TV show. The existence of the show is known but kept secret by their friends.

The Golds argue that the delusion was caused by the rise of new forms of media and an attendant loss of privacy. It’s what you get when a paranoid brain deals with the contemporary social world, whereas perhaps a few hundred years ago these subjects would have been afraid of witches, not TV producers.

It’s a simple picture, and the brain-based concept of mental illness has great power. But culture shapes the brain in ways that makes the simple opposition too stark – London taxi drivers have extra-large hippocampuses, which have grown from use (it keeps a mental map of your surroundings) like the muscles of an athlete.

Over the centuries our brains have been sculpted by our cultural selection just as by natural selection, and mental illness has been shaped accordingly.

At different times, different aspects of a syndrome will predominate, to be succeeded by others as the culture shifts. Historians need to argue about how to apply the labels, but the history of human society is reflected in the ways our minds go wrong.